Wind Turbine Generator System

ABSTRACT

To shorten a startup interval to reach a synchronizing condition, a phase difference and an amplitude difference between the grid voltage and the stator voltage of one phase of a winding are obtained. The difference in amplitude is decreased prior to or in parallel to synchronizing the stator voltage with the grid voltage. The calculated compensation phase compensation value is used as an initial value for synchronizing at the next synchronizing operation.

FIELD OF THE INVENTION

This invention relates to a wind turbine generator system using a doublyfed generator which provides easiness in synchronizing with a gridvoltage.

BACKGROUND OF THE INVENTION

Doubly fed generators can output at their stator an AC voltage havingthe same frequency as that in a grid voltage frequency by exciting arotor winding with a slip frequency by a converter and provide relativefreedom of the rotational frequency and reduction in capacity of aconverter. Japanese laid-open patent application publication No.2000-308398 discloses such a doubly fed generator. Further, due to therelative freedom of the rotational frequency in generation operation andthe reduction in capacity of the converter, the doubly fed generator isused for wind turbine generators. U.S. Pat. No. 6,566,764 discloses sucha wind turbine generator system.

In the conventional wind turbine generator system employing a doubly fedgenerator, a converter has a lower capacity than the generatorfrequently. Thus, when the rotation frequency reaches a predeterminedrange, the wind turbine generator system should frequently repeat startsand stops due to affection of variation in wind power because thegenerator should be operated in parallel to, synchronously with thegrid. This elongates a time interval necessary for synchronous operationwith the grid. Thus, there may be intervals in which power cannot begenerated though wind is blowing. This decreases availability of thewind turbine generator system.

SUMMARY OF THE INVENTION

An aspect of the present invention provides a wind turbine generatorsystem employing a doubly fed generator allowing the system to readilyenter a synchronous operation with the grid.

A further aspect of the present invention provides a wind turbinegenerator system that controls amplitude and a phase of the output toshorten an interval necessary for the doubly fed generator to acquiresynchronism with a grid voltage. Further, an initial value of a positionsensor of the rotor may be automatically compensated.

A further aspect of the present invention provides a wind turbinegenerator system that calculates a phase difference between the gridvoltage and one phase output from a stator to actively compensate thedeviation to shorten the startup interval necessary for reaching thesynchronous condition.

A further aspect of the present invention provides a wind turbinegenerator system in which the number of voltage sensors for the statorcan be reduced.

A further aspect of the present invention provides a wind turbinegenerator system in which an integration value of compensated phaseobtained upon success in synchronism is used as an initial value for thefollowing synchronizing operations.

BRIEF DESCRIPTION OF THE DRAWINGS

The object and features of the present invention will become morereadily apparent from the following detailed description taken inconjunction with the accompanying drawings in which:

FIG. 1 is a skeleton diagram illustrating a circuit configuration of awind turbine generator system according to the present invention;

FIG. 2 is a block diagram illustrating a synchronizing control unitaccording to a first embodiment of the present invention;

FIG. 3 is a block diagram illustrating the synchronizing control unitaccording to the first embodiment;

FIG. 4 is a graphical drawing showing a relation between a detectedphase difference provided by Equation (11) and its approximated valuesprovided by Equation (12) according to the first embodiment;

FIG. 5 is a graphical drawing illustrating a startup operation regardingvoltage and phase matching, in accordance with the first embodiment; and

FIG. 6 is a block diagram illustrating a synchronizing control unitaccording to a second embodiment of the present invention.

The same or corresponding elements or parts are designated with likereferences throughout the drawings.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

Electric wirings and respective sections of a wind turbine generatorsystem for generating an electric power will be described with referenceto FIG. 1. A wind turbine generator Gen according to this embodiment isof a doubly fed type of which three phase outputs from a stator areconnected to a secondary side of a magnet contactor CTT1 switchable inresponse to a switch control signal. A primary side of the magnetcontactor CTT1 is connected to a primary side of a magnet contactor CTT2and to a secondary side of a breaker BR of which primary side isconnected to a grid (an AC line).

The breaker BR has such a function that, upon a flow of an excessivecurrent, a circuit connected to the breaker BR is opened to cut off acurrent flowing therethrough and, upon closing the breaker BR, power issupplied to the control unit CTRL of the wind turbine generator systemand the magnetic contactor CTT1.

The secondary side of the magnet contactor CTT2 is connected to an ACterminal of a converter CNV for cooperative operation through acapacitor Cn and a reactor Ln having a delta connection. On the otherhand, a DC terminal of the converter CNV is connected to a DC terminalof a converter INV for excitation through a smoothing capacitor Cd for aDC current output.

The converter CNV for the cooperative operation and the converter INVfor excitation having power semiconductor switching elements (forexample, thyristors, GTO (gate turn-off thyristor), IGBT (Insulated GateBipolar Transistor), power MOSFET (metal oxide semiconductor fieldeffect transistor), and bi-polar transistors) for converting analternating current into a direct current and vice versa, respectively.

AC terminals of the converter INV for excitation are connected to thesecondary winding terminals of the generator Gen through reactors Lr andcapacitors Cr. A rotor of the generator Gen is coupled to a pinwheel(wind turbine, windmill) 101 for wind turbine power generation via atransmission gear unit and rotates in accordance with power of wind.

Wirings and units for controlling the generated power will be described.Three phase voltages and three phase currents of the primary side of thebreaker BR are converted, by voltage sensors (transducers) PTs andcurrent sensors (transducers) CTs, into low voltage signals Vs and Issupplied to the control unit CTRL. Three phase currents of the secondaryside of the magnet contactor CTT1 (between the magnet contactor CTT2 andthe converter CNV) are converted by current sensors (transducers) CTninto low voltage signals In indicative of values of the three phasecurrents, which are applied to the control unit CTRL. A rotationfrequency and a position of the generator Gen are detected by an encoder102 to output a phase signal PLr (pulse train) supplied to the controlunit CTRL. A voltage of the capacitor Cd connected to the DC terminalsof the converters CNV and INV is converted by a voltage sensor into alow voltage signal Vd supplied to the control unit CTRL.

The control unit CTRL will be described with reference to FIGS. 2 and 3.

The control unit CTRL controls the magnet contactors CTT1 and CTT2, andthe converters INV and CNV by outputting signals Sg1, Sg2, Pulse_inv,and Pulse_cnv. The converter CNV for the cooperative operation issubjected to DC voltage control for keeping a DC voltage Edc at thesmoothing capacitor Cd constant and to a system zero reactive power(power factor is one) from the control unit CTRL around when the windturbine generator system is running and the generator Gen is coupled tothe grid with the magnet contactor CTT1.

More specifically, the breaker BR is closed, which turns on the controlunit CTRL for operation. After that, the control unit CTRL closes themagnet contactor CTT2, so that the control unit CTRL and the converterCNV keep the DC voltage Edc constant. When a start command is applied tothe control unit CTRL, the control unit CTRL and the converter INVexecutes a startup operation as shown in FIG. 5. When the stator voltageagrees with the grid voltage in amplitude and phase, a CTT1 contactcommand and a switching signal SG0 are outputted to make the CTT1 closeand switch a switch SW (mentioned later). The magnet contactor CTT2 iskept close during the startup and the generation operation.

Thus, when the DC output voltage at the capacitor Cd drops due to use ofthe DC power by the converter INV for exciting, the converter CNV forcooperative operation charges the smoothing capacitor Cd using the ACpower to maintain the DC voltage Edc constant. On the other hand, whenthe DC output voltage at the capacitor Cd increases due to charging thecapacitor Cd by the converter INV for exciting, the converter CNV forcooperative operation converts the DC power into the AC power todischarge the smoothing capacitor Cd to maintain the DC voltage Edcconstant.

The converter CNV for the cooperative operation will be described withreference to FIG. 3. The AC voltage detection value Vs is supplied to athree-to-two-phase converter 32 trs. An output of the three-to-two-phaseconverter 32 trs is supplied to a phase detector THDET, a circuit dqtrs,and a synchronizing controller SYNC. The phase detector THDET calculatesa phase signal THs tracking the grid voltage by, for example, a phaseLocked Loop system (PLL) and supplies the phase signal THs to a three-totwo-phase coordinate converter 32 dqtrs and the two-to-three-phasecoordinate converter dq23 trs. The DC voltage command value Eref and theDC voltage detection value Edc are supplied to a DC voltage regulatorDCAVR comprising a proportional integrator. The DC voltage regulatorDCAVR controls a d-axis current command value (effective componentcurrent command value) Idnstr in its output so as to make zero thedeviation of the detected value from the inputted command value, theoutput being supplied to a current controller 1-ACR.

The three-to-two-phase coordinate converter 32 dqtrs calculates a d-axiscurrent detection value Idn (active current), a q-axis current detectionvalue Iqn (reactive current) from the inputted current In using Equation(1) to supply the d-axis current detection value Idn to the currentcontroller 1-ACR and the q-axis current detection value Iqn to a currentcontroller 2-ACR.

$\begin{matrix}{\begin{bmatrix}{Idn} \\{Iqn}\end{bmatrix} = {\begin{bmatrix}{{{Iu} \cdot {\cos (0)}} + {{Iv} \cdot {\cos \left( {2\; {\pi/3}} \right)}} + {{Iw} \cdot {\cos \left( {4\; {\pi/3}} \right)}}} \\{{{Iu} \cdot {\sin (0)}} + {{Iv} \cdot {\sin \left( {2\; {\pi/3}} \right)}} + {{Iw} \cdot {\sin \left( {4\; {\pi/3}} \right)}}}\end{bmatrix}{\quad\begin{bmatrix}{\cos ({THs})} & {\sin ({THs})} \\{- {\sin ({THs})}} & {\cos ({THs})}\end{bmatrix}}}} & (1)\end{matrix}$

The current controller 1-ACR controls the d-axis voltage command valueVdn0 at its output so as to make zero a deviation of the d-axis currentdetection value Idn from the d-axis current command value Idnstr tosupply the d-axis voltage command value Vdn0 to an adder 301. Similarly,the current controller 2-ACR controls the q-axis voltage command valueVqn0 at its output so as to make zero a deviation of the q-axis currentdetection value Iqn from the q-axis current command value (0) to supplythe q-axis voltage command value Vqn0 to the adder 301. The currentcontrollers 1-ACR and 2-ACR may comprise, for example, a proportionalintegrator.

The three-to-two phase converter 32 trs calculates, using Equation (2),from the inputted voltage Vs, a d-axis voltage detection value (a phasecomponent agreeing with the grid voltage vector) Vds and a q-axisvoltage detection value (a quadrature (orthogonal) component of thed-axis voltage detection value Vds) Vqs to supply them to the adders 301and 302.

The adder 301 adds the d-axis voltage command value Vdn0 to the d-axisvoltage detection value Vds to supply the result to the two-to-threephase coordinate converter dq23 trs. Similarly, the adder 302 adds theq-axis voltage command value Vqn0 to the q-axis voltage detection valueVqs to supply the result to the two-to-three phase coordinate converterdq23 trs.

The two-to-three phase coordinate converter dq23 trs receives the phasesignal THs and the results Vdn and Vqn of the adders 301 and 302 andcalculates the voltage command values Vun, Vvn, and Vwn with Equations(2) and (3) to supply them to a PWM (pulse width modulation) operatorPWMn.

$\begin{matrix}{\begin{bmatrix}{Va} \\{Vb}\end{bmatrix} = {\begin{bmatrix}{\cos ({THs})} & {- {\sin ({THs})}} \\{\sin ({THs})} & {\cos ({THs})}\end{bmatrix}\begin{bmatrix}{Vdn} \\{Vqn}\end{bmatrix}}} & (2) \\{\begin{bmatrix}{Vun} \\{Vvn} \\{Vwn}\end{bmatrix} = {\begin{bmatrix}{\cos (0)} & {\sin (0)} \\{\cos \left( {2\; {\pi/3}} \right)} & {\sin \left( {2\; {\pi/3}} \right)} \\{\cos \left( {4\; {\pi/3}} \right)} & {\sin \left( {4\; {\pi/3}} \right)}\end{bmatrix}\begin{bmatrix}{Va} \\\; \\{Vb}\end{bmatrix}}} & (3)\end{matrix}$

The PWM operator PWNn calculates from the inputted voltage commands Vun,Vvn, and Vwn, a gate signal Pulse_cnv for in a PWM method, turning onand off the n power semiconductor switching elements forming theconverter CNV for the cooperative operation.

Control of the converter INV for excitation to generate gate signalswill be described. The phase signal PLr indicative of the rotationfrequency and the position of the rotor is applied to a rotation phasedetector ROTDET. The rotation phase detector ROTDET converts the pulsePLr into a phase signal through counting the pulse PLr and resets thephase signal with an index pulse (z phase pulse in a ABZ system encoder)generated once per rotation to supply the phase signal RTH ranging 0° to360° without overflow to an adder 303.

The adder 303 adds the phase signal RTH to an output phase signal LTH ofthe synchronizing controller SYNC to supply a phase signal TH to anexcitation phase operator SLDET together with the phase signal THs. Theexcitation phase operator SLDET adds the phase signal TH to the phasesignal THs and multiplies the result by the number of pairs of poles ofthe generator to output a phase signal THr indicative of an electricalangle frequency of the rotor of the generator Gen.

A power operator PQCAL receives the d-axis current Ids detected byconverting the system current Is with the Equation (1), having the samedirection as the U-phase vector of the grid voltage, the q-axis currentIqs which is a quadrature (orthogonal) component of the U-phase vectorof the grid voltage, the d-axis voltage detection value Vds, and theq-axis voltage detection value Vqs, to calculate an effective electricpower Ps and a reactive electric power Qs of the system in accordancewith Equations (4) and (5).

Ps=3(Vds×Ids+Vqs×Iqs)  (4)

Qs=3(−Vds×Iqs+Vqs×Iqs)  (5)

An effective electric power controller APR receives the effectiveelectric power Ps and an output power command Pref of the wind turbinegenerator system to generate a torque current command value Iq0 so as tomake zero a deviation of the electric power detection value Ps from theelectric power command value Pref. A reactive electric power controllerAQR receives the reactive power Qs and an output power command Qref togenerate an excitation current command value Id0 so as to make zero adeviation of the electric detection value Qs from the electric powercommand value Qref. The effective electric power controller APR and thereactive electric power controller AVR comprise, for example, aproportional integrator. Current command values Iq0 and Id0 of theoutputs of the effective power controller APR and the reactive powercontroller AQR are supplied to the switch SW.

The switch SW determines whether the outputs of the effective electricpower controller APR and reactive electric power controller AQR or avalue of zero and an output of the voltage controller AVR are used as atorque current command value and an excitation current command value.

The switch SW outputs the latter (the value of zero for the torquecurrent command value and the output of the voltage controller AVR forthe excitation current command value) before the magnet contactor CTT1is made close, i.e., in a synchronizing operation for synchronizing thestator output voltage with the grid voltage. After contact of theelectromagnet contactor CTT1, the switch SW outputs the former (theoutput of the effective power controller APR and the reactive powercontroller AQR).

The voltage controller AVR is supplied with the amplitude value Vgpk ofthe stator voltage Vg as a feedback value and a value Vsref obtained byfiltering an amplitude value of the grid voltage Vs as a command valueto produce an output of the excitation current command value Id1 so asto make zero a deviation of the amplitude value of the stator voltage Vgfrom the command value Vsref to supply the output to the switch SW. Thevoltage controller AVR may comprise, for example, a proportionalintegrator controller. The voltage controller AVR operates in the opencondition of the magnet contact CTT1 to calculate the command value ofthe excitation current flowing through the secondary windings of thegenerator Gen from the converter INV for excitation in order to equalizethe amplitude values of the stator voltages of the generator Gen to theamplitude values of the grid voltages.

The three-to-two phase coordinate converter 32 dqtrs calculates a d-axiscurrent detection value Idr (excitation current component) and a q-axiscurrent detection value Iqr (torque current component) with Equation (6)from the inputted current Ir and the phase THr of the rotor to supplythe d-axis current detection value Idr to a current controller 4-ACR andthe q-axis current detection value Iqr to a current controller 3-ACR.

$\begin{matrix}{\begin{bmatrix}{Idr} \\{Iqr}\end{bmatrix} = {\begin{bmatrix}{{{Iu} \cdot {\cos (0)}} + {{Iv} \cdot {\cos \left( {2\; {\pi/3}} \right)}} + {{Iw} \cdot {\cos \left( {4\; {\pi/3}} \right)}}} \\{{{Iu} \cdot {\sin (0)}} + {{Iv} \cdot {\sin \left( {2\; {\pi/3}} \right)}} + {{Iw} \cdot {\sin \left( {4\; {\pi/3}} \right)}}}\end{bmatrix}{\quad\begin{bmatrix}{\cos ({THs})} & {\sin ({THs})} \\{- {\sin ({THs})}} & {\cos ({THs})}\end{bmatrix}}}} & (6)\end{matrix}$

The current controller 4-ACR controls its output of the d-axis voltagecommand value Vdr so as to make zero a deviation of the d-axis currentdetection value Idr from the d-axis current command value Id1 or Id0.Similarly, the current controller 3-ACR controls its output of theq-axis voltage command value Vqr so as to make zero a deviation of theq-axis current detection value Iqr from the q-axis current command valueIq1 or Iq0. The current controllers may comprise, for example, aproportional integrator.

The d-axis voltage command value Vdr and the q-axis voltage detectionvalue Vqr are applied to a two-to-three-phase coordinate converter dq23trs, which calculates a voltage command values Vur, Vvr, and Vwr as itsoutputs from the phase signal THr and the inputted values with Equations(7) and (8) to output them to a PWM operator PWMr.

$\begin{matrix}{\begin{bmatrix}{Va} \\{Vb}\end{bmatrix} = {\begin{bmatrix}{\cos ({THr})} & {- {\sin ({THr})}} \\{\sin ({THr})} & {\cos ({THrs})}\end{bmatrix}\begin{bmatrix}{Vdr} \\{Vqr}\end{bmatrix}}} & (7) \\{\begin{bmatrix}{Vur} \\{Vvr} \\{Vwr}\end{bmatrix} = {\begin{bmatrix}{\cos (0)} & {\sin (0)} \\{\cos \left( {2\; {\pi/3}} \right)} & {\sin \left( {2\; {\pi/3}} \right)} \\{\cos \left( {4\; \pi} \right)} & {\sin \left( {4\; {\pi/3}} \right)}\end{bmatrix}\begin{bmatrix}{Va} \\\; \\{Vb}\end{bmatrix}}} & (8)\end{matrix}$

The PWM operator PWMr generates, from the inputted voltage commands Vur,Vvr, and Vwr, gate signals Pulse_inv for, in a pulse width modulation(PWM) method, turning on and off the m semiconductor power switchingelements constructing the converter INV for excitation to supply them tothe converter INV.

The synchronizing controller SYNC will be described with reference toFIG. 2.

The synchronizing controller SYNC receives Vα and Vβ obtained bythree-to-two-phase-converting the grid voltage Vs and one phase ofstator voltage of the generator Gen (U-phase voltage Vgu in FIG. 2).

The synchronizing controller SYNC mainly includes two functions, namely,a first function for operating the voltage command value to equalizingthe amplitude value of the stator voltage to that of the grid voltageand a second function for operating a phase compensation value LTH forsynchronizing the phase of the stator voltage with the phase of the gridvoltage. The synchronizing controller SYNC shown in FIG. 2, first,equalizes the voltage amplitude value and second, controls the voltagephase for synchronization.

First, to equalize the amplitude of voltage, the synchronizingcontroller SYNC calculates the amplitude value Vspk of the grid voltagefrom a square root of a sum of squares of the Vα and Vβ and filters thecalculated amplitude value with first-order delay filter FIL to removeripple components to use it as a voltage command value Vsref for thevoltage controller AVR. In this embodiment, only one of stator voltagesis detected, and thus, to obtain the amplitude value of the Vgu, forexample, a maximum amplitude value for one cycle of the grid voltagefrequency (50/60 Hz) is detected as the amplitude value which is used asa feedback value Vgpk in the voltage controller AVR and for an amplitudeagreement judgment unit CMPPK.

The amplitude synchronization judgment unit CMPPK compares the voltageamplitude Vgpk with the voltage command value Vsref. If the differenceis within a predetermined range, an amplitude agreement flag FLG_VG isset “1” In other conditions, the amplitude agreement judgment CMPPKoutputs “0”. The phase synchronizing function operates when theamplitude agreement flag FLG_VG is “1” namely, when the amplitude of thestator voltage substantially agrees with the grid voltage. The term a ofthe grid voltage Vα corresponds to the U phase of the grid voltage.Thus, to synchronize the U phase of the stator voltage Vgu with thephase of the Vα, the difference between the phase of the Vα and the Uphase of the stator voltage Vgu is used.

Here, it is assumed that the grid voltage agrees with the stator voltagein amplitude. Further, if it is assumed that an angular frequency of thegrid voltage is ω0, an angular frequency of the stator voltage is ω1, aphase difference is dTH, and time is t, an absolute value ABSDV of thedifference is calculated by an absolute value operator abs usingEquation (9).

$\begin{matrix}\begin{matrix}{{ABSDV} = {{Va} - {Vgu}}} \\{= {{{Vgpk} \times {\sin \left( {\omega \; {0 \cdot t}} \right)}} - {{Vgpk} \times {\sin \left( {{\omega \; {1 \cdot t}} + {dTH}} \right)}}}}\end{matrix} & (9)\end{matrix}$

Here, the excitation phase THr is obtained by subtracting the rotationphase TH from the grid voltage phase THs, which is so-called a slipfrequency. Thus, excitation with the phase signal THr by the converterINV equalizes the angular frequency ω1 to the angular frequency ω0, sothat the frequency of the stator voltage automatically agrees with thegrid voltage in frequency by the operation of exciting (ω1=ω1).Accordingly, if the amplitudes of voltage are equalized in advance,there is only a difference in phase.

Here, the Equation (9) can be simplified if the voltage amplitudes agreewith each other.

ABSDV=Vgpk× sin(dTH) (after equalization in voltage amplitude)  (10)

An angular converter detects a maximum value in Equation (10) for onecycle of the grid voltage and is divided by Vgpk for normalization.Thus, a phase difference operation value DTH is calculated by Equation(11) to output it.

$\begin{matrix}\begin{matrix}{{DTH} = {{ABSDV}/{Vgpk}}} \\{= {{\sin ({dTH})}\left( {{after}\mspace{14mu} {equalization}\mspace{14mu} {in}\mspace{14mu} {voltage}\mspace{14mu} {amplitude}} \right)}}\end{matrix} & (11)\end{matrix}$

Further, the Equation (11) can be approximated with an Equation (12).

DTH≈dTH  (12)

FIG. 4 represents the detected phase difference DTH obtained by theEquation (11) in the axis of ordinate and the values obtained inaccordance with the Equation (12) in the axis of abscissa throughplotting. As shown in FIG. 4, when the phase difference is small, forexample, when the phase difference in the axis of the abscissa is lowerthan 45°, the values obtained by the Equation (11) substantially agreewith those obtained by the Equation (12). On the other hand, as thephase difference becomes large, the difference between those obtained bythe Equations (11) and (12) becomes large. However, no error occurs in asign (polarity) of the phase difference.

Here, the phase difference DTH has a small error when the voltageamplitudes agree with each other, but has an error when the voltageamplitudes disagree. Thus, in order to provide a surer synchronism inphase even though an error in the voltage amplitude exists, a sign(polarity) of the stator voltage Vgu is judged at a zero-cross point ofthe α term Vα and supplied to the multiplier 202 which multiplies thephase difference DTH by the sign. Inversely, it is possible to detectthe phase relation by judging a sign (polarity) of the α term Vα at azero-cross point of the stator voltage Vgu.

The output of the multiplier 202 indicates the phase difference.However, if this phase difference is outputted as it is, the phase ofthe stator voltage of the generator Gen will rapidly change. Then, anintegrator 201 with limitation is provided for the phase differencedetection value DTH to output a phase difference compensation value LTH.

In the integrator 201 with a limiter, the input is limited by a limiterLMT, and the integrator 201 integrates the output of the limiter LMT,which prevents a rapid phase change in the stator voltage. Further, thevalue of the integration upon success in synchronizing is stored to beused as an initial value at the following startup operations.

The phase synchronism judging unit CMPTH outputs a flag FLG_TH of “1”when the detected phase difference DTH is within a predetermined rangearound zero level and the flag FLG_TH of “1” in other conditions. Theoutput of the phase synchronism judging unit CMPTH is supplied to atiming device DLY, which supplies the synchronizing signal SYN to thesystem controller SYS in FIG. 3 when flag FLG_TH of “1” is continuouslyoutputted for a predetermined interval. Thus, the timing device DLYoutputs the synchronizing flag SYN when the flag FLG_TH of “1” issuccessively outputted for the predetermined interval. In response tothe synchronizing signal SYN, the system controller SYS outputs signalsSg0 and Sg1 to operate the switch SW and the electromagnet contact CTT1.

FIG. 5 shows waveforms in a startup operation according to the structureshown in FIGS. 2 and 3. At a startup timing (t=0), the magnet contactorCTT1 is open, and only the converter CNV for cooperative operationoperates. Then, equalizing the stator voltage to the grid voltage inamplitude is effected prior to the phase synchronizing operation. Whenthe amplitude of the stator voltage substantially agrees with that ofthe grid voltage, the phase of the stator voltage is changed to make thedifference in phase small by controlling the slip frequency to controlthe inverter INV.

When the phase of the stator voltage substantially agrees with that ofthe grid voltage, the system controller SYS supplies the switchingsignal Sg0 to the switch SW and a close command to the magnet contactorCTT1.

More specifically, in FIG. 5, first the frequency of the stator voltageis equalized to that of the grid voltage by calculating the slipfrequency, second the amplitude of the stator voltage is equalized, andfinally, the phase of the stator voltage is synchronized with the gridvoltage by controlling the converter INV.

Second Embodiment

A wind turbine generator system according to a second embodiment hassubstantially the same structure as that of the first embodiment exceptthe part of the synchronizing controller SYNC as shown in FIG. 6. Morespecifically, amplitudes and phases are detected with discrete Fouriertransformers 601 a and 601 b, which receive the α term Vα of the gridvoltage (50/60 Hz) and the stator voltage Vgu to calculate fundamentalcomponents to be compared. In this structure, only fundamentalcomponents are used for judgment for synchronizing and amplitudeequalization, wherein the voltage amplitudes and phases areindependently detected. As a result, the phase control can be providedin parallel to the amplitude equalization substantially at the sametime.

As described above, the stator voltage Vgu is equalized in amplitude toand synchronized with the grid voltage, using one phase of grid voltage.Thus, the use of only one phase of the grid voltage providesequalization in amplitude to and synchronization with the grid voltagein a short interval. Further, the structure as mentioned above canreduce the number of the voltage sensors on the side of the stator andshorten the startup interval because of the active equalizing andsynchronizing operation.

In the second embodiment, the limiter is provided for the phasecompensation value to prevent a rapidly change in a phase of the statorvoltage. Further, the integration value upon success in synchronizationis stored and used as an initial value of the next startup operation.Thus, the initial value of the rotor position sensor is automaticallycompensated as well as a predetermined value is used as the initialvalue. This further shortens the startup interval. In addition, if theinitial phase position may be deviated by maintenance of the rotorposition sensor, the initial value is automatically compensated, whichmakes the maintenance easy. Further, the synchronizing operation can becompleted within one second from start of exciting the secondarywinding.

As mentioned above, according to the present invention, the doubly fedgenerators or motors can be easily linked with the grid, this systemaccording to the present invention is applicable to linkage for, inaddition to the wind turbine generator system, various generators usingpower sources (hydro-electric power stations, flywheel generators,engine generators or the like).

1-15. (canceled)
 16. A wind turbine generator system including a doublyfed generator including a stator and a rotor, comprising: a switcharranged on a side of the stator for linkage with a grid; a converterfor exciting a secondary winding of the doubly fed generator, the rotorbeing coupled to a wind turbine for generating electric power; acontroller configured to control the converter; first detecting meansfor detecting a stator voltage at the switch on a side of the stator;second detecting means for detecting a grid voltage at the switch on aside of the grid; third detecting means for detecting a rotation phaseof the doubly fed generator; wherein the controller controls excitationof the secondary winding to control an amplitude and a phase of thestator voltage; and controlling means for closing the switch when thephase of the stator voltage is identical with the phase of the gridvoltage and the stator voltage is identical with the grid voltage inamplitude.
 17. The wind turbine generator system as claimed in claim 16,wherein the controller generates a torque current command after thecontrolling means closes the switch (see page 9, line 25 to p 11, line10).
 18. The wind turbine generator system as claimed in claim 16,wherein the controller generates the torque current command foreffective power control and reactive power control after the controllingmeans closes the switch.